NETAD LESSON 2

OSI

It's a 7-layered framework for designing and implementing computer networks.

Open Systems Interconnection.

OSI

7

How many layers does OSI model have

Layer 1 – Physical Layer

The lowest layer of the OSI reference model. It is responsible for the actual physical connection between the devices. This layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together. Common physical layer devices are Hub, Repeater, Modem, and Cables.

Functions of the Physical Layer

Bit Synchronization, Bit Rate Control, Physical Topologies, Transmission Mode

Bit Synchronization

The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.

Bit Rate Control

The Physical layer also defines the transmission rate i.e. the number of bits sent per second.

Physical Topologies

Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus topology, star topology, or mesh topology.

Transmission Mode

Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full duplex.

Layer 2 – Data Link Layer (DLL)

is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address.

Frame

Packet in the Data Link layer is referred to as

Switches and Bridges

are common Data Link Layer devices.

Logical Link Control (LLC) & Media Access Control (MAC)

The Data Link Layer is divided into two sublayers

Functions of the Data Link Layer

Framing, Physical Addressing, Error Control, Flow Control, Access Control.

Framing

 is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.

Physical Addressing

After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.

Error Control

The data link layer provides this mechanism in which it detects and retransmits damaged or lost frames.

Flow Control

The data rate must be constant on both sides else the data may get corrupted thus, it coordinates the amount of data that can be sent before receiving an acknowledgment

Access Control

When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

Layer 3 – Network Layer

works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender and receiver’s IP address are placed in the header

Packet

Segment in the Network layer is referred to as

Layer 4 – Transport Layer

provides services to the application layer and takes services from the network layer. It is responsible for the end-to-end delivery of the complete message. It also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found. Protocols used in this Layer are TCP, UDP NetBIOS, PPTP.

Segments

The data in the transport layer is referred to as

Functions of the Transport Layer

Segmentation and Reassembly, Service Point Addressing

Segmentation and Reassembly

This layer accepts the message from the (session) layer and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.

Service Point Addressing

deliver the message to the correct process, the transport layer header includes a type of address called _______ or port address. 

Layer 5 – Session Layer

 is responsible for the establishment of connections, management of connections, terminations of sessions between two devices. It also provides authentication and security. Protocols used in this Layer are NetBIOS, PPTP.

Functions of the Session Layer

Session Establishment, Maintenance, and Termination

Synchronization

Dialog Controller

Layer 6 – Presentation Layer

is also called the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. Protocols used in thisLayer are JPEG, MPEG, GIF, TLS/SSL, etc.

Functions of the Presentation Layer

Translation, Encryption/ Decryption, Compression

Translation

For example, ASCII to EBCDIC.

Encryption/ Decryption

 translates the data into another form or code.

ciphertext

The encrypted data is known as the

plain text

decrypted data is known as

Compression

Reduces the number of bits that need to be transmitted on the network.

Layer 7 – Application Layer

At the very top of the OSI Reference Model stack of layers, we find this layer which is implemented by the network applications. These produce the data to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. Protocols used in this layer are SMTP, FTP, DNS, etc.

Functions of the Application Layer

Network Virtual Terminal (NVT), File Transfer Access and Management (FTAM), Mail Services, Directory Services

Network Virtual Terminal (NVT)

It allows a user to log on to a remote host.

File Transfer Access and Management (FTAM)

This application allows a user to access files in a remote host, retrieve files in a remote host, and manage or control files from a remote computer.

Mail Services

Provide email service.

Directory Services

This application provides distributed database sources and access for global information about various objects and services.

TCP Model

is a fundamental framework for computer networking. This model defines how data is transmitted over networks, ensuring reliable communication between devices. It consists of four layers: the Link Layer, the Internet Layer, the Transport Layer, and the Application Layer. Each layer has specific functions that help manage different aspects of network communication, making it essential for understanding and working with modern networks.

Transmission Control Protocol/Internet Protocol

TCP Means

Department of Defense (DoD)

TCP/IP was designed and developed by _________ in the 1960s

main work of TCP/IP

to transfer the data of a computer from one device to another. The main condition of this process is to make data reliable and accurate so that the receiver will receive the same information which is sent by the sender. To ensure that, each message reaches its final destination accurately, the TCP/IP model divides its data into packets and combines them at the other end, which helps in maintaining the accuracy of the data while transferring from one end to another end.

Layers of TCP/IP Model

• Application Layer

• Transport Layer(TCP/UDP)

• Network/Internet Layer(IP)

• Network Access Layer

Network Access Layer

represents a collection of applications that require network communication. This layer is responsible for generating data and initiating connection requests. It operates on behalf of the sender to manage data transmission,

Internet or Network Layer

This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for the logical transmission of data over the entire network.

IP, ICMP, ARP

The main protocols residing at the Internet or Network layer

Internet Protocol (IP)

responsible for delivering packets from the source host to the destination host by looking at the IP addresses in the packet headers.

IPv4

is the one that most websites are using currently.

IPv6

 is growing as the number of IPv4 addresses is limited in number when compared to the number of users.

Internet Control Message Protocol (ICMP)

Address Resolution Protocol (ARP)

Its job is to find the hardware address of a host from a known IP address

TCP/IP Transport Layer

exchange data receipt acknowledgments and retransmit missing packets to ensure that packets arrive in order and without error. End-to-end communication is referred to as such.

Transmission Control Protocol (TCP)

Applications can interact with one another using ___ as though they were physically connected by a circuit. ___ transmits data in a way that resembles character-by-character transmission rather than separate packets. A starting point that establishes the connection, the whole transmission in byte order, and an ending point that closes the connection make up this transmission.

User Datagram Protocol (UDP)

The datagram delivery service is provided by ___ , the other transport layer protocol. Connections between receiving and sending hosts are not verified by ___. Applications that transport little amounts of data use ___ rather than TCP because it eliminates the processes of establishing and validating connections.

Application Layer

 combines the functions of three layers from the OSI model: the Application, Presentation, and Session layers. This layer is analogous to the transport layer of the OSI model. It is responsible for end-to-end communication and error-free delivery of data. It shields the upper-layer applications from the complexities of data.

HTTP and HTTPS, SSH, NTP

the three main protocols present in the application layer are

HTTP

It is used by the World Wide Web to manage communications between web browsers and servers.

HTTPS

It is a combination of HTTP with SSL(Secure Socket Layer). It is efficient in cases where the browser needs to fill out forms, sign in, authenticate, and carry out bank transactions.

Secure Shell (SSH)

It is a terminal emulations software similar to Telnet. The reason it is preferred is because of its ability to maintain the encrypted connection. It sets up a secure session over a TCP/IP connection.